In typical olefin plants, such as illustrated in U.S. Pat. No. 7,223,895, there is a front-end demethanizer for the removal of methane and hydrogen followed by a deethanizer for the removal of ethane, ethylene and C2 acetylene. The bottoms from this deethanizer tower consist of a mixture of compounds ranging in carbon number from C3 to C6. This mixture may be separated into different carbon numbers, typically by fractionation.
The C3 cut, primarily propylene, is removed as product and is ultimately used for the production of polypropylene or for chemical synthesis such as propylene oxide, cumene, or acrylonitrile. The methyl acetylene and propadiene (MAPD) impurities must be removed either by fractionation or hydrogenation. Hydrogenation is preferred since some of these highly unsaturated C3 compounds end up as propylene thereby increasing the yield.
The C4 cut consisting of C4 acetylenes, butadiene, iso- and normal butenes, and iso- and normal butane can be processed in many ways. A typical steam cracker C4 cut contains the following components in weight %:
TABLE 1Typical C4 cut components and weight percentages.C4 AcetylenesTraceButadiene33%1-butene15%2-butene 9%Isobutene30%Iso- and Normal Butanes13%
Typically, the butadiene and C4 acetylenes are removed first. This can be accomplished by either hydrogenation or extraction. If extraction is employed, the remaining 1-butene and 2-butene remain essentially in the same ratio as that of the initial feedstock. If hydrogenation is employed, the initial product from butadiene hydrogenation is 1-butene. Subsequently, hydroisomerization occurs within the same reaction system changing the 1-butene to 2-butene. The extent of this reaction depends upon catalyst and reaction conditions within the hydrogenation system. However, it is common practice to limit the extent of hydroisomerization in order to avoid “over hydrogenation” and the production of butanes from butenes. This would represent a loss of butene feedstock for downstream operations. The butenes remaining in the mixture consist of normal olefins (1-butene, 2-butene) and iso-olefins (isobutene). The balance of the C4's in the mixture consists of both iso- and normal-butanes from the original feed plus what was produced in the hydrogenation steps and any small quantity of unconverted or unrecovered butadiene.
The butenes have many uses, and in many processes it is desirable to have isomerization of double bonds within a given molecule. Double bond isomerization is the movement of the position of the double bond within a molecule without changing the structure of the molecule. This is different from skeletal isomerization where the structure changes (most typically representing the interchange between the iso-form and the normal form). Skeletal isomerization proceeds by a completely different mechanism than double bond isomerization. Skeletal isomerization typically occurs using a promoted acidic catalyst.
Double bond isomerization is an equilibrium limited reaction. For the equilibrium between 1 butene and 2-butene (cis and trans), the interior olefin (2 butene) is favored at lower temperatures. Starting with either pure butene-1 or pure butene-1 or mixtures thereof, the reaction will move to the equilibrium ration of butene-2 to butene-1. There are two primary reaction routes. One is hydroisomerization where the reaction occurs over typically a noble metal catalyst in the presence of hydrogen at lower temperature and the other is non-hydroisomerization where the reaction occurs generally at higher temperatures over basic metal oxide catalysts and no hydrogen is used.
Double bond hydroisomerization can occur in a hydrogenation reactor. The hydroisomerization reaction uses small quantities of hydrogen over noble metal catalysts (such as Pt or Pd) and occurs at moderate temperatures while the latter is hydrogen free and typically employs basic metal oxide catalysts at higher temperatures. Double bond hydroisomerization usually takes place at moderate temperatures to maximize the interior olefin (2-butene for example as opposed to 1-butene) as the thermodynamic equilibrium favors the interior olefin at lower temperatures. This technology is usually preferred when there is a need to produce an internal olefin for a downstream process. Ethylenolysis of 2-butene to make propylene is such a reaction. The ethylenolysis (metathesis) reaction is 2-butene+ethylene→2 propylenes. Mixed normal butenes (1- and 2-butenes) are typically used as the feed for the metathesis reaction and hydroisomerization is employed upstream of the metathesis reaction to maximize 2-butene in the feed.
However, double bond isomerization can also occur independently without the use of hydrogen in either an independent isomerization reactor or in conjunction with metathesis and typically employs basic metal oxide catalysts at higher temperatures. While interior olefins remain the predominant normal butene in the mixture as the temperature is increased, the formation of the alpha olefin (1 butene) by equilibrium is increased. The use of the basic metal oxide catalyst in the absence of hydrogen eliminates the production of the paraffin by hydrogenation that would result from a hydroisomerization system.
Ethylenolysis (metathesis) of 2 butene occurs at high temperature for example 300 C over a metathesis catalyst. However, only 2-butene participates in this metathesis reaction. The metathesis reaction of 1-butene with ethylene is considered to be a non-productive reaction as the products of this metathesis reaction are essentially the same as the reactants. Therefore, it is advantageous to convert as much of the 1-butene to 2-butene, simultaneously during metathesis to thus maximize the production of propylene. Under these conditions, non-hydroisomerization is employed and typically the basic metal oxide isomerization catalyst is physically mixed with the metathesis catalyst to allow both reactions to proceed simultaneously.
Conventional metathesis with isomerization involves the reaction of mixed normal butenes (both 1-butene and 2-butene) with ethylene to produce propylene, as described above. These reactions occur in the presence of a group VIA or VIIA metal oxide metathesis catalyst, either supported or unsupported in combination with basic metal oxide isomerization catalysts. Typical catalysts for metathesis are tungsten oxide supported on silica or rhenium oxide supported on alumina. Examples of catalysts suitable for the metathesis of olefins are described in U.S. Pat. No. 6,683,019, for example.
However, it is common to employ a double bond isomerization catalyst within the metathesis reactor to shift 1-butene to 2-butene and allow for continued reaction. Typical double bond isomerization catalysts include basic metal oxides (Group IIA), either supported or unsupported. Hydroisomerization is particularly not preferred since at the elevated temperatures of the reaction, the required hydrogen would saturate some fraction of the olefin reactants to paraffins thus reducing the product yields. For example, U.S. Pat. No. 6,875,901 discloses a process for the isomerization of olefins using a basic metal oxide catalyst, such as a high purity magnesium oxide catalyst, which may be in the form of powder, pellets, extrudates, and the like. Magnesium oxide and calcium oxide are examples of such double bond isomerization catalysts that may be physically admixed with the metathesis catalyst. No equivalent co-catalyst exists for the skeletal isomerization of isobutene to normal butene. In the case of a conventional metathesis system employing both a metathesis catalyst and a co-mixed double bond isomerization catalyst, the butadiene must be removed to a level of less than 500 ppm to avoid rapid fouling of the double bond isomerization catalyst. The metathesis catalyst itself can tolerate butadiene levels up to 10,000 ppm.
Isobutene is typically removed from the feedstock prior to the metathesis reaction step. The reaction of isobutene with ethylene is non-productive and metathesis reaction with itself and/or other C4's is limited in the presence of excess ethylene. Non-productive reactions essentially occupy catalyst sites but produce no product. If allowed to remain in the feed to the metathesis unit, the concentration of this non-reactive species would buildup creating capacity limitations. Options for isobutene removal include reacting it with methanol to produce methyl tertiary butyl ether (MTBE) or separating the isobutene from the butenes by fractionation. U.S. Pat. No. 6,358,482 discloses the removal of isobutene from the C4 mixture prior to metathesis. This scheme is further reflected in U.S. Pat. Nos. 6,075,173 and 5,898,091. U.S. Pat. No. 6,580,009 discloses a process for the production of propylene and hexene from a limited ethylene fraction. For molar ratios of ethylene to butenes (expressed as n-butenes) from 0.05 to 0.60, a raffinate II stream is used as the C4 feedstock. A raffinate II stream is by definition a stream following isobutene removal. Isobutene removal from the C4 stream may also be accomplished by employing a combined catalytic distillation hydroisomerization deisobuteneizer system to both remove the isobutene and recover n-butenes at high efficiency by isomerizing the 1-butene to 2-butene, as described in U.S. Pat. No. 5,087,780.
High temperature double bond isomerization catalysts are also used for double bond isomerization alone, not in the presence of a metathesis catalyst and/or ethylene. For example, 1-butene is a valuable co-monomer for the production of certain grades of polyethylene. 1-Butene can be produced via the isomerization of 2 butene coupled with fractionation as described in U.S. Pat. No. 6,875,901. Furthermore as described in U.S. Pat. No. 6,727,396, such an isomerization catalyst is useful in the isomerization of internal hexene isomers (2 and 3 hexene) to 1-hexene. 1-hexene is also a valuable co-monomer for polyethylene. In this case, the metathesis takes place between 1 butene and itself (1 butene+1 butene→ethylene+3 hexene). This reaction uses similar metathesis catalysts as referenced above but critically, the feed to the metathesis step must be highly concentrated 1 butene. The basic metal oxide isomerization catalyst is used as described in U.S. Pat. No. 6,875,901 to produce the stream of highly concentrated 1 butene. The 1 butene is then subjected to metathesis alone specifically avoiding a isomerization function in that step. The resultant 3 hexene is then subjected to a separate high temperature—non hydroisomerization double bond isomerization step The advantage of such isomerization is the favorable equilibrium at higher temperatures for the alpha olefin and the lack of hydrogen present to hydrogenate olefins to paraffins.
The metathesis catalysts and the double bond isomerization catalysts are quite sensitive to poisons. Poisons include water, CO2, oxygenates (such as MTBE), sulfur compounds, nitrogen compounds, and heavy metals. It is common practice to employ guard beds upstream of the metathesis reaction system to insure the removal of these poisons. It does not matter if these guard beds are directly before the metathesis reaction system or further upstream as long as the poisons are removed and no new poisons are subsequently introduced. Typical guard bed adsorbents are alumina and or activated alumina. It is also possible to use basic metal oxides such as magnesium oxide and/or calcium oxide as guard bed materials. At low temperatures, they have the capacity to adsorb water and react with oxygenates such as methanol to form water and carbon dioxide. The water formed is subsequently adsorbed by the other basic oxide sites.
Metathesis reactions are very sensitive to the location of the olefin double bond and the stereo-structure of the individual molecules. A pair of olefins adsorbs on the surface and exchange double bond positions with the carbon groups on either sides of the double bonds. Metathesis reactions can be classified as productive, half productive or non-productive. As described above, non-productive reactions result in essentially no reaction taking place. When the double bonds shift with metathesis reaction, the new molecules are the same as the originally adsorbed molecules thus no productive reaction occurs. This is typical for reactions between symmetric olefins or reactions between ethylene and alpha olefins. If fully productive reactions occur, new products are generated no matter which orientation the molecules occupy the sites. The reaction between ethylene and 2-butene to form two propylene molecules is a fully productive reaction. Half productive reactions are sterically inhibited. If the pair of olefins adsorb in one orientation, when the double bonds shift, new products are formed. Alternately if they adsorb in a different steric configuration, when the bonds shift, the identical olefins are formed and thus no new products are formed. The various metathesis reactions proceed at different rates (a fully productive reaction is usually faster than a half productive reaction) and with different weight selectivities to propylene. Table 2 summarizes the reactions between ethylene and various butenes and the reactions between the butenes themselves.
The reactions listed represent the base reaction with ethylene (reaction 1, 4 and 5) as well as the reactions between the various C4 olefins. It is especially important to make a distinction between the selectivity to propylene from total C4 olefins (including isobutene) and the selectivity to propylene from the normal C4 olefins involved in the reaction. The reaction of isobutene with 2-butene (reaction 6) produces propylene and a branched C5 molecule. For this reaction, propylene is produced at 37.5 weight % selectivity from total C4's (similar to reaction 2) but at a 75 weight % selectivity from normal C4's (2-butene). For the purposes of definitions, conventional metathesis is defined as the reaction of the C4 olefin stream with ethylene. However, the C4 stream can also react in the absence of ethylene as a feedstock. This reaction is called auto or self metathesis. In this case, reactions 2, 3, 6, and 7 are the only possible reactions and will occur at rates dependent upon the feedstock composition.
TABLE 2Wt. % SelectivityWt. % Selectivity(C3H6 from(C3H6 fromNo.ReactionTypeRatetotal C4s)n-C4s)12-butene + ethylene →FullyFast1001002 propyleneProductive(Conventional Metathesis)21-butene + 2-butene →FullyFast37.537.5Propylene + 2-penteneProductive31-butene + 1-butene →HalfSlow00Ethylene + 3-hexeneProductive4Isobutene + Ethylene →Non-No——No reactionproductiveReaction51-butene + ethylene →Non-NoNo reactionproductiveReaction6Isobutene + 2-butene →FullyFast37.575Propylene + 2-methyl 2-buteneProductive7Isobutene + 1-butene →HalfSlow00ethylene + 2-methyl 2 penteneproductive
In conventional metathesis for propylene production, the focus is to maximize reaction 1 to produce propylene. This will maximize the selectivity to propylene. As such, excess ethylene is used to reduce the extent of the reactions of butenes with themselves (reactions 2, 3, 6, and 7). The theoretical ratio is 1/1 molar or 0.5 weight ratio of ethylene to n-butenes, but it is common in conventional metathesis to employ significantly greater ratios, typically, 1.3 or larger molar ratio to minimize reactions 2, 3, 6 and 7. Under conditions of excess ethylene, and due to the fact that both isobutene and 1-butene do not react with ethylene (see reactions 4 and 5), two process sequences are employed. First, the isobutene is removed prior to metathesis. If isobutene is not removed, it will buildup as the n-butenes are recycled to achieve high yield. Second, 1-butene is isomerized to 2-butene by including a double bond isomerization catalyst such as magnesium oxide admixed with the metathesis catalyst. Note that this catalyst will not cause skeletal isomerization (isobutene to normal butenes) but only shift the double bond from the 1 position to the 2 position. Thus by operating with excess ethylene, eliminating isobutene from the metathesis feed prior to reaction, and employing a double bond isomerization catalyst, reaction 1 is maximized.
As described above, magnesium oxide catalysts may be mixed with metathesis catalysts for performing both double-bond isomerization and metathesis in the same reactor. In such a system, the magnesium oxide serves two functions. First, the magnesium oxide acts as a guard bed, adsorbing various oxygenates and water to protect the metathesis catalyst. At the higher temperatures of the metathesis reaction, the capacity for adsorption is much lower than at temperatures closer to ambient but this function provides a valuable second poison adsorption step following the bulk poison removal via guard beds as mentioned above. Second, as described above, the reaction of ethylene with 1-butene is non-productive; as 1-butene essentially does not react with ethylene, 1-butene will buildup in the recycle stream. In order to avoid 1-butene buildup, double-bond isomerization catalysts, such as magnesium oxide, may be used to isomerize the 1-butene to 2-butene as the 2-butene is depleted during the reaction.
Double-bond isomerization catalysts, such as magnesium oxide, are currently commercially used in the form of tablets having an effective diameter of about 5 mm. As used herein, effective diameter refers to the diameter that non-spherical shaped particles would have if it were molded into a sphere. These tablets exhibit good isomerization activity when processing butenes alone. However, such tablets exhibit activity for isomerization of 1-butene to 2-butene only for a short time in the presence of ethylene. Further, their performance is progressively worse as the number of reaction cycles increase. After several regeneration/reaction cycles, their activity for isomerization is low. This performance shortfall may lead to a rapid buildup of 1-butene in the system over time, limiting reactor performance by hydraulically limiting the recycle, and limiting the overall conversion of butenes to propylene that can be obtained economically. A similar loss of activity is experienced when operating these catalysts as double bond isomerization catalysts alone for the production of the terminal olefin from the interior olefin.
It is well known in the industry that smaller sized catalyst particles exhibit better performance during the reaction cycles. This is due to the reduction of internal mass transfer resistance. This allows the reactants to have greater access to the catalyst sites. By reducing the mass transfer resistances, improved reactivity is achieved. However, the loss of activity with regeneration cycles is not improved. The loss of activity as a result of regenerations is due not to simple mass transfer limitations as a function of effective diameter but to the loss of surface area of the catalyst particle (of any size) due to sintering created by the higher temperatures required for coke removal for example.
Some attempts have been made to improve the performance of magnesium oxide catalysts. For example, U.S. Pat. No. 6,875,901 discloses improvements to the deactivation rate of magnesium oxide isomerization catalysts by limiting certain impurities, such as phosphorous, sulfur, transition metals, etc. Deactivation in the presence of ethylene, however, remains problematic.
As described above, there remains a need for basic metal oxide double-bond isomerization catalysts that may improve the overall performance of the metathesis process, increasing propylene yield and decreasing 1-butene recycle purge. There is also a need for an improved version of these catalysts for the simple double bond isomerization of interior olefins to terminal olefins, for example butene-2 to butene-1 or hexene 2 or hexene-3 to hexene-1. For both of these systems there is a need to reduce the cycle to cycle deactivation thus maintaining higher activity over the complete catalyst life cycle.